Motor driver

ABSTRACT

A motor driver drives a motor composed of a rotor and plural-phase windings that generate a magnetic field for rotating the rotor, including: a plurality of transistors that operate as switches for supplying current to the windings; a position detector operable to detect a rotational position of the rotor, based on a terminal voltage of each winding; and a switching controller operable to have performed a switching method to turn the transistors to an ON state or to an OFF state for controlling the rotor at the predetermined speed by means of the position detector, wherein the switching controller further controls so as to force each of the transistors into the OFF state for a predetermined duration in a predetermined cycle, and the position detector detects only while the switching controller forcedly keeps the transistor in the OFF state.

TECHNICAL FIELD

The present invention relates to a motor driver that performs PWM (pulsewidth modulation) sensorless driving.

BACKGROUND ART

The following describes a conventional motor driver.

FIG. 14 shows the structure of a conventional motor driver.

In the figure, a rotor 1010 has a magnetic field unit achieved bypermanent magnets, and generates a rotation force according to mutualaction of windings 1011, 1012, and 1013. A power supplier 1020 iscomposed of three upper power transistors and three lower powertransistors, and supplies power to the windings 1011, 1012, and 1013. Aposition detector 1030 compares each of terminal voltages V1, V2, andV3, which are each from one terminal of the windings 1011, 1012, and1013, respectively, with a common voltage Vc, and outputs a detectionpulse signal FG in accordance with the result of comparison. A commandunit 1040 outputs a speed command signal EC for controlling speed of therotor 1010. In accordance with the signal EC, the switching controller1050 outputs a PWM signal Wp for having the upper power transistors ofthe power supplier 1020 perform PWM operation. In accordance with thedetection signal pulse FG and the PWM signal Wp, a distributioncontroller 1060 outputs upper distribution control signals N1, N2, andN3 and lower distribution control signals M1, M2, and M3 for controllingpower distribution to the windings 1011, 1012, 1013. Accordingly, thepower supplier 1020 supplies power to the windings 1011, 1012, and 1013,and has the motor perform PWM sensorless driving.

A further structure is disclosed in Japanese Patent ApplicationPublication No. 2001-346394 (p. 18, paragraph no. 0051), for havingposition detection performed stably in order to eliminate instability inaccelerated turning operation caused by lag in position detection.

A problem exists in these conventional motor drivers in that startupfailure occurs easily. Startup failure occurs because the rotor 1010 isunstable in terms of position and rotates slowly at the initial startup,and therefore the back EMF (electromagnetic force) voltage that isinduced in the windings 1011, 1012, and 1013 is low. Consequently, theposition sensor 1030, which detects position based on the comparisonresults of the terminal voltage V1, V2, and V3 of the windings 1011,1012, and 1013 with the common voltage Vc, detects erroneously.

Particularly, when having the motor perform PWM driving, induced noisethat is characteristic of PWM operation is superimposed on the terminalvoltage in the detection phase. As a result, the probability of theposition sensor 1030 detecting erroneously further increases due to thissuperimposed noise.

A further conventional technique that attempts to deal with this problemis a method that fixes the position of the rotor in startup according tomagnetic pull in a specific phase. However, this method is problematicbecause the motor driver takes longer to start up due to the additionaltime required to fix the position of the rotor.

In view of the stated problems, the object of the present invention isto provide a motor driver that enables stable PWM sensorless startup inPWM sensorless driving, taking into consideration the effects of noisethat is characteristic of PWM operation.

DISCLOSURE OF THE INVENTION

In order to achieve the stated object, the present invention is a motordriver that drives a motor composed of a rotor and plural-phase windingsthat generate a magnetic field for rotating the rotor, including: aplurality of transistors that operate as switches for supplying currentto the windings; a position detector operable to detect a rotationalposition of the rotor, based on a terminal voltage of each winding; anda switching controller operable to have performed a switching method toturn the transistors to an ON state or to an OFF state for controllingthe rotor at the predetermined speed by means of the position detector,wherein the switching controller further controls so as to force each ofthe transistors into the OFF state for a predetermined duration in apredetermined cycle, and the position detector detects only while theswitching controller forcedly keeps the transistor in the OFF state.

With this structure, detection of the position of the rotor is performedonly while the switching controller forcedly keeps the transistors inthe OFF state. In this period, erroneous position detection due toinduced noise that is characteristic of PWM operation can be prevented,and, as a result, startup failure due to erroneous detection can beprevented. In other words, stable PWM sensorless startup is achieved.

In particular, when having the motor perform PWM driving according tothe transistors performing high-frequency switching between the ON stateand the OFF state, induced noise caused by current change in PWMoperation is superimposed on terminal voltage of the winding that isbeing used for position detection. Position detection is more likely tobe erroneous if performed using the terminal voltage on which thisinduced noise has been superimposed. Therefore, the present inventionhas a structure in which position detection is performed in segments inwhich PWM operation is forced off.

Furthermore, the present invention may be characterized in that therotor has permanent magnets, each winding is mounted on a stator, themotor driver further includes a DC power unit that is a power supplysource, the plurality of transistors is composed of a group oftransistors that operate as switches for supplying power from oneterminal of the DC power unit to one end of each winding, and a group oftransistors that operate as switches for supplying power from anotherterminal of the DC power unit to another end of each winding, and theswitching controller performs the control for forcing the OFF state withrespect to at least transistors of one of the groups.

With this structure, it is sufficient for the switching controller tocontrol only one of the groups of transistors. Since it is not necessaryto control both the groups of transistors, the circuit structure can besimplified.

Furthermore, the present invention may be characterized in that theposition detector stops detecting for a predetermined period commencingat a point at which a change from the ON state to the OFF state occurswhen the switching controller forces the OFF state, and thepredetermined duration relating to the switching controller forcing theOFF state is longer than the predetermined period.

With this structure, the adverse effect of ringing, which occurs duringthe predetermined period, on detection can be avoided.

Furthermore, the motor driver of the present invention may furtherinclude: a rotation speed determiner operable to determine whether ornot a rotation speed of the rotor is at least a predetermined speed,wherein, when the rotation speed is determined to be at least thepredetermined speed, the position detector detects at least while atransistor is in the ON state.

With this structure, when the rotation speed is at least thepredetermined speed, the effect of induced noise that accompaniescurrent change according to PWM operation is reduced. Therefore,position detection can be performed with even more stability.

Furthermore, the present invention may be characterized in that when therotation speed is determined to be at least the predetermined speed, theswitching controller stops forcing the OFF state.

With this structure, while the rotation speed is less than thepredetermined speed, in other words, during startup when the speed isone at which erroneous detection occurs easily, erroneous detection canbe prevented by forcedly turning the transistors to the OFF state. Onthe other hand, when the rotation speed reaches at least thepredetermined speed, erroneous detection is relatively unlikely to occurcompared with the when the rotation speed is less than the predeterminedspeed. Furthermore, a problem occurs that, when the rotation speed is atleast the predetermined speed, rotation becomes unstable easily becauseof fluctuations in driving current due to a wide forced-off segment.Therefore, the present invention is structured such that the switchingcontroller stops forcedly turning the transistors to the OFF state whenthe rotation speed reaches at least the predetermined speed. Thissuppresses fluctuations in driving current, and consequently achievesstable rotation.

Furthermore, the present invention may be characterized in that theposition detector (a) when the rotation speed is determined not to be atleast the predetermined speed, stops detecting for a first periodcommencing at a point at which a change from the ON state to the OFFstate occurs when the switching controller forces the OFF state, and (b)when the rotation speed is determined to be at least the predeterminedspeed, stops detecting for a second period commencing at a point atwhich a transistor changes from the OFF state to the ON state, and thepredetermined duration relating to the switching controller forcing theOFF state is longer than the first period.

With this structure, when the rotation speed is less than thepredetermined speed, position detection is suppressed during the initialfirst period commencing at the point at which the transistors areforcedly changed from the ON state to the OFF state. This avoids adverseeffect of ringing which is prone to occurring in this period.Furthermore, when the rotation speed reaches at least the predeterminedspeed, position detection is suppressed during an initial second periodcommencing at the point at which the transistors change from the OFFstate to the ON state. This avoids adverse effect of ringing which isprone to occurring during this period.

Furthermore, the present invention may be characterized in that therotation speed determiner performs the determination based on the resultof the detection by the position detector.

With this structure, the rotation speed determiner is able to determinethe rotation speed using the detection result from the positiondetector, without special structure being required for rotation speeddetermination. Therefore, the circuit structure can be simplified.

Furthermore, the present invention may be characterized in that theswitching controller turns a predetermined one of the transistors to theON state in each constant period, turning the transistor to an OFF statefor a predetermined period directly before turning the transistor to theON state.

Furthermore, the present invention may be characterized in that thepredetermined cycle in which the switching controller forces the OFFstate is no greater than 1/20000 seconds.

Furthermore, the present invention may be characterized in that theposition detector detects the position of the rotor by comparing aterminal voltage of each winding with a center tap voltage of allwindings or with a pseudo-center tap voltage of the terminal voltages ofthe windings.

Furthermore, the present invention may be characterized in that thecycle in which the switching controller forces the OFF state includes asegment in which a driving current of each winding is 0, and theposition detector detects during the segment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of a motor driver of a first embodiment;

FIG. 2 shows detailed structure of a position detector 30;

FIG. 3 shows detailed structure of a switcher 50;

FIG. 4 shows the relationship between the waveform of each signal of aswitching controller 52;

FIG. 5 shows the structure of a motor driver of a second embodiment;

FIG. 6 shows detailed structure of a position detector 30A;

FIG. 7 shows the structure of a motor driver of a third embodiment;

FIG. 8 shows detailed structure of a switcher 50;

FIG. 9 shows the relationship between the waveform of each signal of theswitching controller 52 in a first position detection mode;

FIG. 10 shows the relationship between the waveform of each signal ofthe switching controller 52 in a second position detection mode;

FIG. 11 shows the structure of a motor driver of a fourth embodiment;

FIG. 12 shows detailed structure of a switching controller 52A;

FIG. 13 shows the relationship between the waveform of each signal ofthe switching controller 52A; and

FIG. 14 shows the structure of a conventional motor driver.

BEST MODE FOR CARRYING OUT THE INVENTION

The following describes embodiments of the present invention withreference to the drawings.

First Embodiment

FIG. 1 shows the structure of a motor driver of the first embodiment.

In the figure, a rotor 10 has a magnetic field unit attached theretowhich generates a plurality of magnetic field poles using permanentmagnets. Three phase windings 11, 12, and 13 are mounted on stators,which are stationary parts, and are arranged so as to be electricallyshifted by 120 degrees with respect to the rotor 10. One terminal ofeach winding is connected to a power supplier 20, and the otherterminals are commonly connected. The three phase windings 11, 12, and13 generate three phase magnetic flux according to three phase drivingvoltages I1, I2, and I3, and generate driving power according by mutualaction with the rotor 10. This rotates the rotor 10 and a disc 1 mountedon the rotor 10.

A DC power source 5 is the source of power. The negative terminal of theDC power source 5 is connected to earth potential and the positiveterminal of the DC power source 5 supplies required DC voltage Vm. Thepositive terminal of the DC power source 5 is commonly connected via acurrent detector 51 to current input terminals of three upper powertransistors 21, 22, and 23. The current output terminals of the upperpower transistors 21, 22, and 23 are connected to the power supplyterminals of the three phase windings 11, 12, and 13, respectively.Furthermore, the negative terminal of the DC power source 5 is commonlyconnected to current output terminals of three lower power transistors25, 26, and 27. The current output terminals of the lower powertransistors 25, 26, and 27 are connected to power supply terminals ofthe three phase windings 11, 12, and 13, respectively. In addition,upper power diodes 21 d, 22 d, and 23 d are connected anti-parallel withthe upper transistors 21, 22, and 23, respectively, and lower powerdiodes 25 d, 26 d, and 27 d are connected anti-parallel with the lowertransistors 25, 26, and 27, respectively. Note that N-channel fieldeffect power transistors are used for the upper power transistors 21,22, and 23 and the lower power transistors 25, 26, and 27, and parasitediodes that are formed in anti-parallel connection with each of theN-channel field effect power transistors are used as the upper powerdiodes 21 d, 22 d, and 23 d, and the lower power diodes 25 d, 26 d, and27 d, respectively.

The power supplier 20 is composed of the upper power transistors 21, 22,and 23, the lower power transistors 25, 26, and 27, the upper powerdiodes 21 d, 22 d, and 23 d, and the lower power diodes 25 d, 26 d, and27 d. The upper power transistors 21, 22, and 23 open and close powersupply paths between the positive terminal of the DC power source 5 andthe power supply terminal of each of the three-phase windings 11, 12,and 13 in accordance with upper power distribution control signals N1,N2, and N3 from a distribution controller 60, thereby forming currentpaths that supply positive current of the driving currents I1, I2, andI3 to the three-phase windings 11, 12, and 13. Upper power controldistribution signals N1, N2, and N3 serve as digital PWM signals in eachpower distribution segment, according to a PWM signal Wp from aswitching controller 52. In other words, the upper power transistors 21,22, and 23 perform high-frequency switching operation. The lower powertransistors 25, 26, and 27 open and close power supply paths between thenegative terminal of the DC power source 5 and the negative terminal ofeach of the three-phase windings 11, 12, and 13 in accordance with lowerpower distribution control signals M1, M2, and M3 from the distributioncontroller 60, thereby forming current paths that supply negativecurrent of the driving currents I1, I2, and I3 to the three-phasewindings 11, 12, and 13. Note that structure and operation of theswitching controller 52 are described in detail later.

The position sensor 30 detects the rotational position of the disc 1 andthe rotor 10, and outputs a detection pulse signal FG corresponding tothe detection result. FIG. 2 shows detailed structure of the positionsensor 30. The position sensor 30 is composed of four input resistors31, 32, 33, and 34, three voltage comparison circuits 35, 36, and 37, anoise removal circuit 38, and a detection circuit 39. The terminalvoltages V1, V2, and V3 respectively generated in one terminal of eachof the three phase windings 11, 12, and 13 are respectively input, witha common connection center tap voltage Vc, to the voltage comparisoncircuits 35, 36, and 37 via the input resistors 31, 32, and 33. Thevoltage comparison circuits 35, 36, and 37 compare the terminal voltagesV1, V2, and V3 with the center tap voltage Vc, and output voltagecomparison signals C1, C2, and C3 in accordance with the comparisonresults. The noise removal circuit 38 removes switching noise caused byhigh-frequency switching operation from the voltage comparison signalsC1, C2, and C3 of the voltage comparison circuits 35, 36, and 37, andoutputs voltage comparison signals C1R, C2R, and C3R obtained as aresult of the noise removal. Note that a mask signal Wm from theswitching controller 52 is used in the noise removal. This mask signalWm is described in detail later. Next, the detection circuit 39 detectsthe position of the disc 1 and the rotor 10 using the noise-removedvoltage comparison signals C1R, C2R, and C3R from the noise removalcircuit 38 and detection window signals WIN 1–6 from the distributioncontroller 60, and outputs a detection pulse signal FG corresponding tothe detection result. The detection pulse signal FG is input to thecommand unit 40 and the distribution controller 60.

Detection windows WIN 1–6 are described here. The detection windowsignals WIN 1–6 are output from the distribution controller 60, andcorrespond to the zero cross of the rising edge and falling edge of theback EMF voltage induced in the three-phase windings 11, 12, and 13 inrespective non-power distribution phases. For example, the detectionwindow signal WIN 1 is a window for detecting rising edge zero cross ofthe back EMF voltage of the winding 11, and the detection window signalWIN 2 is a window for detecting falling edge zero cross of the back EMFvoltage of the winding 13. In this way, the detection window signals WIN1–6 are electrically shifted by a 60 degree electrical angle.

The command unit 40 is composed of a speed control circuit that controlsso that the disc 1 and the rotor 10 rotate at a predetermined speed. Thecommand unit 40 detects the rotation speed of the disc 1 and the rotor10 according to the detection pulse signal FG from the position detector30, and outputs a speed command signal Ac that varies in accordance witha difference between the detected speed and a target rotation speed.

A switcher 50 is composed of the current detector 51, the switchingcontroller 52, and a forced-off signal generator 53. FIG. 3 showsdetailed structure of the switcher 50. The current detector 51 iscomposed of a current detection resistor 110, and outputs a currentdetection signal Ad that is proportionate to the current supplied to thethree-phase windings 11, 12, and 13 from the positive terminal of the DCpower 5 via the upper power transistors 21, 22, and 23.

The forced-off control generator 53 outputs a forced-off signal Wo thatreaches a low level every constant cycle To. This forced-off signal Wois input into the switching controller 52. The switching controller 52compares the current detection signal Ad from the current detector 51with the speed command signal Ac from the command unit 40, outputs a PWMreset signal Pr in accordance with the comparison result, and outputsthe PWM signal Wp and the mask signal Wm that correspond to the PWMreset signal Pr. The PWM signal Wp is input into the distributioncontroller 60, and the mask signal Wm is input into the noise removalcircuit 38 of the position detector 30. The PWM signal Wp is for havingthe upper power transistors 21, 22, and 23 of the power supplier 20perform high frequency switching operation (PWM operation).

Note that the motor driver of the first embodiment may instead bestructured such that the current detector 51 is between the negativeterminal of the DC power supply 5 and the lower power transistors 25,26, and 27.

The switching controller 52 includes a comparison circuit 111, areference trigger generation circuit 112, a PWM signal generationcircuit 113, an AND gate 115, and a mask signal generation circuit 116.The comparison circuit 111 compares the current detection signal Ad fromthe current detector 51 with the speed command signal Ac from thecommand unit 40, and outputs a PWM reset signal Pr that varies inaccordance with the comparison result. Specifically, when the currentdetection signal Ad becomes higher than the speed command signal Ac, thePWM reset signal changes from the low level to the high level. Thereference trigger generation circuit 112 outputs a reference triggersignal Ps at a constant cycle Tp. Specifically, 1/Tp is a value between20 kHz and 500 kHz, inclusive. The PWM signal generation circuit 113outputs a basic PWM signal Wb according to the PWM reset signal Pr fromthe comparison circuit 111 and the reference trigger signal Ps from thereference trigger generation circuit 112. The relationship between thereference trigger signal Ps, the PWM reset signal Pr, and the basic PWMsignal Wb is shown in FIG. 4. The basic PWM signal Wb changes to thehigh level on the rising edge of the reference trigger signal Ps in theconstant cycle Tp, and changes to the low level according to the risingedge of the PWM reset signal Pr. In this way, the basic PWM signal Wb isa PWM signal that corresponds to the result of the comparison of thecurrent detection signal Ad and the speed command signal Ac. In otherwords, the duty of the basic PWM signal Wb varies in accordance with thespeed command signal Ac from the command unit 40. Specifically, when theactual rotation speed of the disc 1 and the rotor 10 is slower than thetarget rotation speed, the speed command signal Ac from the command unit40 is high, and the ON duty of the basic PWM signal Wb increases.Conversely, when the actual rotation speed of the disc 1 and the rotor10 is faster than the target rotation speed, the speed command signal Acfrom the command unit 40 is low, and the ON duty of the basic PWM signalWb decreases. Furthermore, when the target rotation speed and the actualrotation speed of the disc 1 and the rotor 10 are substantially equal,the speed command signal Ac from the command unit 40 has a value thatcorresponds to the target rotation speed, and the ON duty of the basicPWM signal Wb also has a value that substantially corresponds to thetarget rotation speed.

As has been described, the rotation speed of the disc 1 and the rotor 10is controlled by detecting the actual rotation speed using the detectionpulse signal FG of the position detector 30, outputting a speed commandsignal Ac that varies in correspondence with the difference with thetarget rotation speed, and varying the ON duty of the basic PWM signalWb in accordance with the speed command signal Ac.

The forced-off signal generator 53 outputs a forced-off signal Wo thatforcedly sets the upper power transistors 21, 22, and 23 of the powersupplier 20 to OFF every constant cycle To. This forced-off signal Wo isinput into one of the input terminals of the AND gate 115 of theswitching controller 52, and the basic PWM signal Wb from the PWM signalgenerator 113 is input into the other input terminal. The AND gate 115performs AND synthesis and outputs the PWM signal Wp. The relationshipbetween the waveforms of signals of the switching controller 52 is shownin FIG. 4. The upper power transistors 21, 22, and 23 of the powersupplier 20 perform high frequency switching operation according to thisPWM signal Wp. In other words, in addition to high frequency switchingaccording to the basic PWM signal Wb, forced-off operation is performedforcedly every constant cycle To, according to the forced-off signal Wo.At this time, since the current is cut off every constant cycle Toaccording to the forced-off signal Wo, a problem of noise arises if therepetition frequency 1/To of the forced-off signal Wo is within anaudible frequency range. For this reason, it is preferable to set therepetition frequency 1/To of the forced-off signal Wo to be outside theaudible frequency range, specifically, to 200 kHz or above. In otherwords, it is preferable that To is no greater than 1/20000 seconds. Notethat forced-off operation according to the forced-off signal Wo is notlimited to being performed every constant cycle Tp such as in the motordriver of the present embodiment, but may be performed in arbitrarycycles, or with arbitrary timing.

The PWM signal Wp is also input into the mask signal generator 116. Themask signal generator 116 outputs the mask signal Wm which is forremoving noise, which is caused by high-frequency switching, that issuperimposed on the voltage comparison signals C1, C2, and C3 from thevoltage comparison signals C1, C2, and C3 in the noise removal circuit38 of the position detector 30. The high level segments of the masksignal Wm are segments in which high frequency switching noise ismasked, while the low level segments are segments in which positiondetection is possible. In the motor driver of the present firstembodiment, the mask signal Wm masks in all segments other than theforced-off segment, and further masks for a first predetermined periodTa after forced-off. Consequently, the only segment in which therotational position of the disc 1 and the rotor 10 is able to bedetected is a segment X, which is the forced-off segment A with theexclusion of the first predetermined period Ta. In other words, positiondetection is performed only in the forced-off segment. Note that theforced-off segment A must be set to be longer than the firstpredetermined period Ta that proceeds forced-off (A>Ta).

The distribution controller 60 outputs the upper power distributionsignals N1, N2, and N3 and lower power distribution signals M1, M2, andM3 that vary in correspondence to the detection pulse signal FG from theposition detector 30, thereby controlling power distribution from theupper power transistors 21, 22, and 23 and the lower power transistors25, 26, and 27 of the power supplier 20 to the three-phase windings 11,12, and 13. The upper power distribution signals N1, N2, and N3 are ANDsynthesized with the PWM signal Wp from the switching controller 52. Theupper power transistors 21, 22, and 23 perform high-frequency switchingoperation according to upper power distribution signals N1, N2, and N3(PWM signal WP), and the lower power transistors 25, 26, and 27 performfull-on operation according to the lower power distribution signals M1,M2, and M3. More specifically, while power is being distributed from thewinding 11 to the winding 12, the upper power transistor switch 21performs high-speed switching operation according to the upper powerdistribution signal N1, and the lower power transistor 26 performsfull-on operation according to the lower power distribution signal M2.When the upper power transistor 21 is performing ON operation accordingto the PWM signal Wp, the upper power transistor 21 supplies positivecurrent from the positive terminal of the DC power source 5 to thewinding 11, and the lower power transistor 26 supplies negative currentfrom negative terminal of the DC power source 5 to the winding 12. Next,when the PWM signal Wp is off, since the positive current that wasflowing in the winding 11 attempts to continue flowing due to theinductance action of the winding, positive current is supplied to thewinding 11 by the same-phase lower power diode 25 d. In this way, PWMoperation is performed. Furthermore, as described earlier, thedistribution controller 60 also outputs the detection window signals WIN1–6 that vary according to the detection pulse signal FG from theposition detector 30.

The motor driver of the present first embodiment performs PWM sensorlessdriving with the above-described structure. Generally, since it isnecessary to detect the rotational position of the disc 1 and the rotor10 in sensorless driving, sensorless driving of a motor is performed byproviding a non-power distribution segment, in other words, by providinga segment in which same-phase upper and lower power transistors in thepower supplier 20 are OFF, and performing zero cross detection of theback EMF voltage induced in the corresponding winding in that segment.However, since the rotor is unstable in terms of position and rotatesslowly at initial startup, the back EMF voltage induced in thethree-phase windings 11, 12, and 13 is low, and, consequently, positiondetection is difficult. This causes the conventional problem of start upfailure in sensorless driving. In particular, it was found that, whenhaving the motor perform PWM driving, induced noise that accompanieschanges in current according to PWM driving is superimposed on thedetection phase terminal voltage. Consequently, in sensorless startup,the position is erroneously detected due to the effect of induced noise,and startup failure occurs easily. In this way, induced noise isgenerated accompanying current changes in PWM operation, and the adverseeffects thereof are particularly great in position detection duringinitial startup.

Here, induced noise is described. Induced noise is voltage generated asa result of a change in current according to PWM operation. Describinginduced noise in more detail, in the power supplier 20 of FIG. 1, theupper power transistor 21 is made to perform PWM operation, and thelower power transistor 27 is made to perform full-on operation. In thisstate, power is distributed from the winding 11 to the winding 13, andthe detection phase is winding 12. Ordinarily, when the motor is notrotating, the center tap voltage Vc at the common connection point andthe terminal voltage V2 of the detection phase (winding 12) are equal,and the difference voltage therebetween should be 0. However, when PWMoperation is performed, induced noise, which is a phenomenoncharacteristic of PWM operation, is superimposed on the detection phaseterminal voltage V2 with respect to the center tap voltage Vc. Inducednoise is voltage generated accompanying current change according to PWMoperation, however, the value of the induced noise is the opposite oneof positive or negative to the current change amount. Furthermore, thelevel of induced noise varies depending on the current change amount.

One method for startup is to fix the position of the rotor according tomagnetic pull in a specific phase, and having the motor start after theposition of the disc 1 and the rotor 10 has been fixed. Although stablesensorless startup can be achieved by first fixing the initial positionbefore startup in this way, fixing the initial position takes time. Forthis reason, a method of performing forced synchronized driving atstartup and then switching to sensorless driving is often employed. Witha structure such as that of the present first embodiment in which thepeak of driving current of the three windings 11, 12, and 13 is detectedby the current detector 51, the ON duty of the PWM signal WP directlyafter startup is great, specifically, approximately 100%. In otherwords, position detection is performed almost all the time in PWMoperation ON section. Conventionally, in such a case, induced noise thataccompanies positive voltage change according to PWM operation issuperimposed on the detection phase terminal voltage, causing erroneousdetection of the position and resulting in startup failure.

In light of this problem, the motor driver of the first embodiment has astructure in which an OFF segment is provided and position detection isperformed in the OFF segment. Specifically, the forced-off signalgenerator 53 provided in the switcher 50 outputs a forced-off signal Wothat forcedly turns off the upper power transistors 21, 22, and 23 ofthe power supplier 20 every constant cycle To, and the position detector30 performs position detection only in the forced-off segment. As aresult of position detection operations being performed only in theforced-off segment, position detection is performed with the negativecurrent change. Consequently, the induced noise at the time of positiondetection is of the opposite one of positive and negative to the inducednoise that accompanied current change. Such a structure enables stablePWM sensorless startup.

Note that the forced-off segment A may be any length of time that islonger than the first predetermined period Ta (A>Ta). Specifically, thetime of forced-off segment A is a value of at least 3 μs and no morethan 20 μs. Furthermore, as one example of a way to further reduce theeffects of induced noise, the forced-off segment A may be set to besufficiently long for the driving current to become 0, and positiondetection is performed in the segments when the driving current is 0.Since current change according to PWM operation does not occur in thesegments in which the driving current is 0, induction noise does notoccur. In other words, the influence of induced noise can bedisregarded.

Second Embodiment

FIG. 5 shows the structure of a motor driver of the second embodiment.In the motor driver of FIG. 1, a center tap voltage Vc of a commonconnection point of the terminal voltages V1, V2, and V3 from oneterminal of each of the three-phase windings 11, 12, and 13 is inputinto the position detector 30, and the position detector 30 detects therotational position of the disc 1 and the rotor 10. In contrast, in themotor driver of the present second embodiment, only the terminalvoltages V1, V2, and V3 of the three-phase windings 11, 12, and 13 areinput into a position detector 30A, and the position detector 30Adetects the rotational position without using the center tap voltage Vc.

FIG. 6 shows detailed structure of the position detector 30A. Theterminal voltages V1, V2, and V3 that occur at one terminal of each ofthe three-phase windings 11, 12, and 13 are respectively input into theinput terminals of the voltage comparison circuits 35, 36, and 37 viathe input resistors 31, 32, and 33. A center tap voltage Vci, which is apseudo-center tap voltage of the terminal voltages V1, V2, and V3 thatoccur in one terminal of the three-phase windings 11, 12, and 13, isinput into the other input terminal of each of the voltage comparisoncircuits 35, 36, and 37. The pseudo-center tap voltage Vci is generatedby connecting resistors 34A, 34B, and 34C to the terminal voltages V1,V2, and V3, respectively, and commonly connecting one terminal of eachof the resistors 34A, 34B, and 34C. The voltage comparison circuits 35,36, and 37 directly compare the terminal voltages V1, V2, and V3, whichoccur in one terminal of the three-phase windings 11, 12, and 13, withthe pseudo-center tap voltage Vci. Circuit structure subsequent to thevoltage comparison circuits 35, 36, and 37 is identical to that of thefirst embodiment, and rotational position detection is performed usingonly the terminal voltages V1, V2, and V3 that occur in one terminal ofeach of the three-phase windings 11, 12, and 13.

With the described structure, it is sufficient for three voltages to beinput into the position detector 30A, specifically, the terminalvoltages V1, V2, and V3 that occur in one terminal of each of thethree-phase windings 11, 12, and 13. This is one less input voltage thanin the motor driver of the first embodiment. In other words, the motordriver can be constructed with one less wire from the center tap voltageto the position detector 30A and one less terminal.

Third Embodiment

FIG. 7 shows the structure of the motor driver of the third embodiment.

The structure shown in FIG. 7 differs from that shown in FIG. 1 in thatit additionally includes a rotation speed judgment unit 70.

The detection pulse signal FG from the position detector 30 is inputinto the rotation speed determination unit 70, and the rotation speeddetermination unit 70 determines the rotation speed of the disc 1 andthe rotor 10 using the position detection pulse signal FG. When therotation speed of the disc 1 and the rotor 10 is determined to be atleast a predetermined speed, the rotation speed determination unit 70outputs a high level rotation speed determination signal NS. Note thatthe structure for judging the rotation speed of the disc 1 and the rotor10 is not limited to being a structure in which the position detectionpulse signal FG is used in the judgment. Any other structure by whichthe rotation speed can be judged is possible.

FIG. 8 shows detailed structure of the switcher 50. The basic structureis the same as that in the motor driver of the first embodiment. Therotation speed determination signal NS is input into the forced-offsignal generator 53, the switching controller 52, and the mask signalgenerator 116. Here, “first position detection mode” is used to refer toa detection mode used when the level of the rotation speed determinationsignal NS is low, in other words, position detection during a periodfrom initial startup through to when rotation speed of the disc 1 andthe rotor 10 reaches the predetermined rotation speed. Furthermore,“second position detection mode” is used to refer to a detection modeused when the level of the rotation speed determination signal NS ishigh, in other words, position detection while the rotation speed of thedisc 1 and the rotor 10 is at least the predetermined rotation speed.

FIG. 9 shows the relationship between waveforms of signals in theswitching controller 52 in first position detection mode. In firstposition detection mode, the forced-off signal generator 53 outputs theforced-off signal Wo. Therefore, the PWM signal Wp is an AND output ofthe basic PWM signal Wb and the forced-off signal Wo. The upper powertransistors 21, 22, and 23 of the power supplier 20 perform PWMoperation, which includes forced-off operation, according to this PWMsignal Wp. Meanwhile, the mask signal generator 116 outputs a masksignal Wm that masks during all segments other than the forced-offsegment, and also masks for a first predetermined period Ta after theforced-off segment (This is the same as in the first embodiment.). Inother words, position detection is possible only in a segment X, whichis the forced-off segment A with the exclusion of the firstpredetermined period Ta. Note that the forced-off segment A must be setto be longer than the first predetermined period Ta that proceeds forcedoff (A>Ta).

FIG. 10 shows the relationship between waveforms of signals in theswitching controller 52 in second position detection mode. In secondposition detection mode, the forced-off signal generator 53 outputs ahigh level signal. Therefore, since the PWM signal Wp is the AND outputof the basic PWM signal Wb and the forced-off signal Wo (high level),the PWM signal Wp is the basic PWM signal Wb. The upper powertransistors 21, 22, and 23 of the power supplier 20 perform PWMoperation according to this PWM signal Wp. Meanwhile, the mask signalgenerator 116 is able to perform position detection with respect to thePWM signal Wp in a segment X and a segment Y. Here, the segment X is asegment during a PWM operation OFF segment with the exclusion of thefirst predetermined period Ta directly after proceeding to the OFFsegment, and the segment Y is a segment in the PWM operation ON segmentwith the exclusion of a second predetermined period Tb directlyproceeding to the ON segment.

In this way, stable PWM sensorless startup can be performed in firstmode in which position detection operations are performed only in theforced-off segment from startup through to the rotation speed beingreached. However, since the OFF segment is set relatively widely, thereis a possibility that the driving current will fluctuate and positiondetection will be unstable. Therefore, the motor driver of the thirdembodiment has a structure in which the level of the forced-off signalWo is high when the driving speed exceeds the predetermined drivingspeed, and fluctuations in the driving current are suppressed byprohibiting forced-off operation. In addition, the second detection modein which the mask signal Wm enables position detection during both an ONsegment and an OFF segment in PWM operation is used. Position detectionis performed by switching between the first detection mode and thesecond detection mode depending on the rotation speed of the disc 1 andthe rotor 10.

According to the stated structure, position detection is performed byswitching between the first position detection mode and the secondposition detection mode according to the rotation speed determinationsignal NS that is output from the rotation speed determination unit 70.During the period from startup through to when the predeterminedrotation speed in reached, stable PWM sensorless startup can be achievedbecause position detection is possible only in the forced-off segment.In addition, because forced-off is prohibited when the rotation speed isat least the predetermined rotation speed, and position detection isperformed in a PWM operation ON segment and OFF segment, stableoperation can also be achieved during ordinary driving.

Fourth Embodiment

FIG. 11 shows the structure of the motor driver of the fourthembodiment.

The structure shown in FIG. 11 differs from that shown in FIG. 1 in thatthe switcher 50 has a switching controller 52A.

FIG. 12 shows detailed structure of the switching controller 52A. Thedifference with the switching controller 52 of FIG. 1 is that theswitching controller 52A additionally includes a predetermined-time offsignal generator 117, and the AND gate 115 has three inputs.

FIG. 13 shows the relationship between waveforms of the signals in theswitching controller 52A. In synchronization with the reference triggersignal Ps of the reference trigger generation circuit 112, thepredetermined-time off signal generator 117 outputs a predetermined timeOFF signal Wf for turning off the reference trigger signal Ps for apredetermined period Tf in the constant cycle Tp directly before thereference trigger signal Ps changes to ON. The AND gate 115 ANDsynthesizes the basic PWM signal Wb from the PWM signal generationcircuit 113, the forced-off signal Wo from the forced off signalgenerator 53, and the predetermined-time off signal Wf from thepredetermined-time off signal generation circuit 117, and outputs theresultant synthesized signal as the PWM signal Wp. Other structure isthe same as that of the motor driver of the first embodiment.

With the PWM control of the motor driver of the present invention, theseries of operations consisting of starting PWM operation according tothe reference trigger signal Ps and completing PWM operation bydetecting the peak value is performed every reference trigger signal Ps.In this kind of driver, fluctuations in the driving current are causedby fluctuations in rotation of the load (for example, the disc) that isdriven. Furthermore, there is a tendency for erroneous operation, whichis a phenomenon of a PWM operation of a particular cycle commencingbefore the PWM operation of the previous cycle is complete, in otherwords, “switching loss phenomenon”, to occur.

With the described structure, PWM operation is always performed everyconstant cycle Tp, other than in forced-off segments, and therefore thephenomenon of switching loss can be prevented, and fluctuations indriving current can be reduced. In other words, stable driving isachieved.

INDUSTRIAL APPLICABILITY

The present invention can be used as a motor driving mechanism inoptical disc apparatuses, magnetic disc apparatuses, and the like.

1. A motor driver comprising: plural-phase windings; a plurality oftransistors composed of a first group of transistors that operate asswitches for supplying power from one terminal of a DC power unit to oneend of each winding, and a second group of transistors that operate asswitches for supplying power from another terminal of the DC power unitto another end of each winding; a position detector operable to detect arotational position of a rotor, based on a terminal voltage of eachwinding; a current detector operable to output a current detectionsignal that is proportionate to a current supplied to the plural-phasewindings; a switching controller that includes a forced-off signalgenerator operable to generate a forced-off signal that has a pulsewidth of a predetermined duration in a predetermined cycle thatcorresponds at least to a clock, and a PWM signal generator operable togenerate a basic PWM signal in accordance with a result of comparing thecurrent detection signal and a speed command signal, and is operable togenerate a PWM signal by AND synthesizing the forced-off signal with thebasic PWM signal; and a power distributor operable to generate firstpower distribution signals that correspond to a result of detection bythe position detector, generate second power distribution signals by ANDsynthesizing a power distribution signal that corresponds to a result ofdetection by the position detector with the PWM signal from theswitching controller, and output the generated first power distributionsignals and the generated second power distribution signals, therebycausing the plurality of transistors to perform switching operation,wherein the switching controller further controls such that upper andlower transistors which switch the plural-phase windings are forced intoa compulsory OFF state by means of the forced-off signal included in thePWM signal, and the position detector detects only while the upper andlower transistors are in the compulsory OFF state.
 2. The motor driverof claim 1, wherein the position detector stops detecting for apredetermined period commencing at a point at which a change from the ONstate to the OFF state occurs when the switching controller forces theOFF state, and the predetermined duration relating to the switchingcontroller forcing the OFF state is longer than the predeterminedperiod.
 3. The motor driver of claim 1, further comprising: a rotationspeed determiner operable to determine whether or not a rotation speedof the rotor is at least a predetermined speed, wherein, when therotation speed is determined to be at least the predetermined speed, theposition detector detects at least while the plurality of transistorsare in the ON state.
 4. The motor driver of claim 3, wherein when therotation speed is determined to be at least the predetermined speed, theswitching controller stops forcing the OFF state.
 5. The motor driver ofclaim 3, wherein the position detector (a) when the rotation speed isdetermined not to be at least the predetermined speed, stops detectingfor a first period commencing at a point at which a change from the ONstate to the OFF state occurs when the switching controller forces theOFF state, and (b) when the rotation speed is determined to be at leastthe predetermined speed, stops detecting for a second period commencingat a point at which the plurality of transistors change from the OFFstate to the ON state, and the predetermined duration relating to theswitching controller forcing the OFF state is longer than the firstperiod.
 6. The motor driver of claim 3, wherein the rotation speeddeterminer performs the determination based on the result of thedetection by the position detector.
 7. The motor driver of claim 1wherein the switching controller turns the plurality of transistors tothe ON state of the high-frequency operation, and sets a predeterminedduration for which the transistors are forced into the OFF statedirectly before the transistors are turned to the ON state.
 8. The motordriver of claim 1, wherein the predetermined cycle in which theswitching controller forces the OFF state is no greater than 1/20000seconds.
 9. The motor driver of claim 1, wherein the position detectordetects the position of the rotor by comparing a terminal voltage ofeach winding with a center tap voltage of all windings or with apseudo-center tap voltage of the terminal voltages of the windings. 10.The motor driver of claim 1, wherein the cycle in which the switchingcontroller forces the OFF state includes a segment in which a drivingcurrent of each winding is 0, and the position detector detects duringthe segment.